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4 February 1974 

Most authors and scholars of electric fishes agree that: 

1. The anatomy and physiology of the electric organs of different 
species are diverse; 


2. The diversity amohg species strongly suggest that the electric fe : 

organ-electroreceptor system plays quite different roles in the 
biology of different species. But what is this role is not known &'••• 

.as of today; jfep 


3. Some classification has been made with regard to their transmitting 

electric organ as follows: P 

a. low resting frequency, high standard deviation of interval fn 

lengths and responding to different kinds of stimuli with a I"? 

many-fold increase in frequency and amplitude of the signal £X- 
(for example, Gnathonemus petersii ), pi- 

r-L - ’• 

b. medium resting frequency, medium standard deviation of 
interval lengths, responding to some stimuli with a several- 
fold increase in frequency (for example, Gymnotus carapo ), ' 

c. medium frequency with little or no change in the repetition ^ 

rate of the signal, but with the possibility to stop completely 

the discharge (for example, Gymnarchus niloticus). 


• . 

t'vU-V . . 

d. high resting frequency with low standard deviation, responding 
to a few special stimuli with small shifts in repetition rate of 
the signal (for example, Sternarchus albifrons ), 

e. species with no known response to stimuli with change in 
the repetition rate of the signal. 

4. The discharge of the electric organ has been found to be commanded 
from a pacemaker center in the medulla, in most species studied 
until now. (See Fig. 1.) This train of impulses seems to be re- 
layed 1:1 in a nucleus in the medulla and again in the electromotor 
neurons in the spinal cord to be passed through the spinal nerves 
to the electric transmitting organ. 

Some electric fishes have a very elaborate array of electroreceptors. 

All electric fishes have other sensory receptors mostly part of the lateralis 
line system like: temperature receptors, mechanical receptors and water- 
displacement receptors. These sensory receptors are located in the dermis. 
There are other sensory receptors non-related to the lateralis line like: 
acoustical receptors, chemical receptors, olfactory receptors, taste receptors 
and in some electric fishes optical receptors (vision), not to mention equilibrium 
and stabilizing receptors. 

All these sensory receptors, electric arid nonelectric, may work 
separately or in accordance in a hybrid cross correlation information system 
used in social interaction, feeding, swimming and navigation, in offense or 
defense. Because of the multiplicity of use the sensory system is very com- 
plicated vising neuronal preprocessings and discriminating filtering systems, 
delay -lines, etc. 

Some electric fishes use passive and/or active object detection systems. 
There are some marine and fresh-water fishes which do not have any electric 
transmitting organ, but have an electroreceptor system. (Marine sharks like: 

x ipt^t r r n 


Fig. 1. Connection between the pacemaker in the brain, 
the relay nuclei, the electromotor neurons, the 
electric transmitting organ and the electrore- 
ceptors in electric fishes. 


Negaprion brevirostris , Scilliorhinus canicula , a catfish: Plotosus or the 
eel Angnilla anguilla and freshwater fishes like Clarias. ) 

A classification of electroreceptors can be made from the following 

1. Autorhytmic. This means that they emit a very low amplitude 
signal from less than one to a few millivolts. Depending on the 
particular receptor class it can have a low repetition rate (less 
than 100), a medium repetition rate (from 100 to 500) or a high 
repetition rate (from 500 to 3000), 

2. Nonautorhytmic. 
or we can divide them in: 

3. Synchronous. This means that they respond to the transmitting 
organ in case of a stimulus with a nerve transmission spike rate 
synchronized to the electric organ. 

4. Nonsynchronous. 

Another classification could be used as: 

5. Ampullary organs similar to or identical with the Lorenzini 
ampulla which has a canal lined with a very high resistance 
membrane filled with a jelly and communicating to an ampulla 
with sensory organs.- The jelly can be high, medium or low con- 
ductive and accordingly being acid, neutral or basic. There are 
many kinds of sensory cell formation classes. 

6. Tuberous or mormyromasts, mostly not directly connected to 
the surface of the skin. Every mormyromast may have one, two 
or more sensory cells separately innervated and responding each 
to different amplitudes (levels) of stimuli. 

Maybe a more adequate classification would be one taking into account 
the stimuli to which these electroreceptors respond such as: 


a. Conductivity: conducting or nonconducting objects, the level 

of response being more or less proportional to their conductivity 

b. Direction: response of the sensory cells being conditioned to 
the iact that an object is directed toward or away from the fish; 

c. Form: sharp edges will stimulate some receptors, others will 
be stimulated by round or rounded objects; 

d. Movement: the change in position or form of an object will pro- 
duce a modulation of the autorhytmic impulses of some receptors 
and will be sensed by the fish; 

e. Smoothness or Roughness of objects could also constitute a criteria 
of stimuli classification; 

f. Chemicals in the water may affect the electric receptors and 
their response, we have proof for this. 

Finally coding can be a classification criteria. Lissman and Machin- 
proposed a 

1. "Pulse -frequency-modulation” (like in Gymnarchus niloticus ); 

Watanabe and Bullock proposed a 

2. "Pulse -phase modulation" (like in Eigenmannia virescens ); 

Szabo and Hagiwara analyzed and suggested three other kinds of codings: 

3. "Number coding mechanism" (like in gymnotidsas: Itypopomus 
artedi ), 

4. "Probability coding mechanism" (like in gymotids as: Sternarchus 
albifrons) , 

5. "Latency coding mechanism" (like in mormyrids as: Gnathonemus 


According to the first hypothesis "Pulse -frequency modulation" sen- 
sory information should be conveyed by the frequency of the sensory impulses 
dependent on the pulse of the electric discharges. 

The second hypothesis "Pulse-phase modulation" the sensory coding 
is the result of time relation (the phase) on the sensory impulse following the 
electric organ discharge. 

The third hypothesis called the "Number coding mechanism" supposes 
that the intensity of the electric potential field is coded through a single electro- 
receptor fiber by the number of nerve impulses produced by each electric 
organ pulse. 

In the number four hypothesis, called "Probability coding mechanism, " 
the coding is provided by the probability that each electric organ impulse might 
initiate an impulse in the nerve fiber. 

Finally the fifth hypothesis: "Latency coding mechanism" is explained 
by the fact that certain mormyrid electroreceptors permit a change in latency 
of impulses of the electric organ related to the intensity of the current flowing 
through the receptor. Therefore, the intensity of the potential field can be 
coded by the time relation between electric transmitting organ discharge and 
sensory impulse, the time ranging being as much as 8 milliseconds. For 
variations in the superthreshold field intensity this would be the only mech- 
anism for a sensory organ producing single spikes. The place where the 
latency-shift of the sensory impulse is taking place has not as yet unequivoc- 
ally explained. 

It is worth mentioning that there are electroreceptors connected to 
nerve fibers which would not transmit any impulses without a specific stimulus. 
Other electroreceptors are related to nerve fibers discharging continuously. 


Some, when presented with stimuli, would increase their electric activity 
and others would decrease it. 

The first type of fibers are called phasic fibers, the second type are 
called tonic fibers. Electroreceptors connected to phasic nerve fibers are 
called phasic electroreceptors (i.e., tuberous organs = mormyromasts); 
electroreceptors connected to tonic fibers are called tonic electroreceptors 
(i.e. , ampullary organs like the Lorenzini ampulla ).